The present invention relates to high voltage electrical circuits for generating high frequency switching signals, and more particularly to high voltage switching circuits supported by low voltage transistors.
The symbolic diagram in FIG. 1 illustrates typical structures of prior art voltage converting circuits. The input source voltage (Vi) and input ground voltage (Vss) are connected to a transformer (TF1), and the output voltages (Vout, Vsq) of the transformer (TF1) are connected to filters for further processing. The transformer (TF1) is a static electrical device that transfers energy by inductive coupling between its winding circuits. The input voltages (Vi, Vss) cause a varying current in the primary winding (PL) that creates a varying magnetic flux in the transformer's core and thus a varying magnetic flux through the secondary winding (SL). This varying magnetic flux induces a varying electrical field in the secondary winding that generates the output voltages (Vout, Vsq). At ideal conditions, the relationship between the input and output voltages of the transformer is (Vi−Vss)/(Vout−Vsq)=Np/Ns, where Np is the number of turns in the primary coil, and Ns is the number of turns in the secondary coil of the transformer. By proper selection of Np and Ns, desired output voltages can be generated. Galvanic isolation is also achieved because the input ground voltage (Vss) and the output ground voltage (Vsq) of a transformer can have different voltage values.
The prior art circuit illustrated in FIG. 1 works very well except when the input voltage (Vi) is a slow varying signal (for example, slower than 200 cycles per second). Slow varying input signals require transformers with large inductance values, and the filters also need large capacitors and/or inductors. The required circuit elements, such as transformers and/or filters, used to support low frequency input signals are typically bulky and expensive. FIGS. 2(a-c) illustrates typical prior art solutions for this problem. A switching circuit block (SWC) is placed between the slow varying input voltages (Vin, Vss) and the transformer (TF2), as illustrated by the symbolic diagram in FIG. 2(a). This prior art switching circuit block (SWC) typically comprises a transistor (Mu0) for providing a driving force to pull the output signal (Sw) of the circuit block (SWC) toward Vin when it is enabled and a transistor (Md0) for providing a driving force to pull Sw toward Vss when it is enabled, as illustrated by the symbolic diagram in FIG. 2(b). Control circuits (GCP, GCN) synchronized by an oscillator (OSC0) control the timing of the switching circuits to generate an output signal (Sw) that switches between Vin and Vss, as illustrated by the waveform shown in FIG. 2(c). In this way, the inputs to the transformer (TF2) and/or the filters are high frequency switching signals. Therefore, desired outputs can be generated without using expensive, bulky, low frequency electrical components.
The prior art circuit illustrated in FIG. 2(a-c) works very well except when the input voltage (Vi) is at high voltage. The driving transistors (Mu0, Md0) used by the switching circuit (SWC) in FIG. 2(b) need to tolerate the maximum voltage differences between Vin and Vss. If the input voltage (Vin) is a high voltage signal, then Mu0 and Md0 need to be high voltage transistors. Logic transistors supported by Integrated Circuit (IC) technologies typically operate at voltages lower than 6 volts. Transistors that can tolerate more than 12 volts are typically more expensive because they require additional steps during the manufacturing process. Transistors that can tolerate more than 30 volts typically are not available in IC technologies; they are manufactured by special methods as discrete components, and they are much more expensive and bulky. Transistors that can tolerate more than 100 volts are difficult to manufacture, and are bulky and expensive. The performance of high voltage transistors is typically much lower than the performance of low voltage transistors. Low performance of high voltage transistors limits the achievable frequencies of the switching output signals, thus limiting the capability to use smaller transformers and/or filters. In addition, switching high voltage signals waste more energy than switching low voltage signals. It is therefore highly desirable to support high voltage switching circuits using common low voltage transistors.